1. Field of the Invention
The present invention relates to an apparatus and method for continuously forming slits in a film.
2. Prior Art
Japanese Patent Publication No. Sho 61-11757 discloses an apparatus for forming slits in a film. The outline of this apparatus will be described hereunder.
Referring to FIGS. 17(a) and 17(b), a rotary blade assembly 1 is made of circular blades 12 having plural arcuated blade tips, the blades 12 being arranged around a shaft 11 in parallel with one another and at a predetermined interval or pitch between one blade and the adjacent one. By passing a film 3 through between the rotary blade assembly 1 and a backing roller 2, there is obtained a film having slits formed parallel to the length of the film and according to the shapes of the blades. FIG. 18 illustrates the film 3 in which slits are formed by this apparatus. Reference numeral 35 denotes slits formed in the film 3; and 34a and 34b, non-slit margins (regions near two side ends) of the film, respectively. The backing roller 2 is made of a high molecular material, e.g., rubber and plastics, in order to decrease wear of the blade tips of the expensive rotary blade assembly.
When a continuous operation is performed on a film with this apparatus, satisfactory proper slits cannot be obtained, and there is raised the problem that the film often tears in the subsequent stretching step. The service life of the backing roller made of a high molecular material is very short. Satisfactory slits cannot be formed by the continuous operation of this apparatus possibly because of various errors, e.g., unsatisfactory accuracy in sizes of the rotary blade assembly and the backing roller, an error in the rotation thereof, and non-uniformity in film thickness. For example, the rotating units such as the rotary blade assembly 1 and the backing roller 2 have a dimensional error caused when worked and an error exhibited during the rotation thereof. More specifically, these errors include off centering caused in this conventional apparatus by unsatisfactory perpendicularity of the circular blades to the axis 11 (or unsatisfactory parallelism between one circular blade and the adjacent one), unsatisfactory circularity of the section of the roller, insufficient precision of bearings and unsatisfactory accuracy in installation of this apparatus. Said errors also include non-uniformity of heights of individual blade tips of the circular blades and unevenness of the surface of the roller. Said errors further include considerable variation of the film in thickness. These various errors are superposably affect and cannot be avoided in machining.
When the backing roller has elasticity, an amount of elastic deformation varies depending on the extent of errors, and a push pressure proportional to the elastic deformation also varies accordingly. As a result, when slits are to be formed, the pressing forces at different portions of the film vary, failing to obtain satisfactory slits. The higher the running speed of the films is, the more remarkable the variation in pressing force is.
The surface portion of the backing roller 2 is made of a comparatively soft high molecular material, and it is thus very liable to be damaged and this damage will be quickly aggravated during the operation of this apparatus. Thus, in anticipation of such variation as above to some extent, the pressing force is generally adjusted. However, if the pressing force is set to be smaller so that the damage is less worsened, then satisfactory slits will not be formed.
When the damage or injure in the surface portion of the backing roll 2 is substantial, V- or U-shaped cuts 25 are found in the surface portion as indicated in FIG. 19(a). Accordingly, to form satisfactory slits in a film, the pressing force of the rotary blade assembly and the like must always be adjusted.
In the above apparatus, the blade tips are heated. As is indicated in FIG. 19(b), as the damage or cuts 25 in the backing roller 2 are deepened, the area and time of contact between each of the blade tips of the circular blades 12 and the film 3 are increased so that the film is melted and the film fails to form trim slits. In addition, the film 3 tends to easily adhere to the blade tips when they are heated and, therefore, it cannot be fed uniformly whereby satisfactory slits are not formed thus causing the film to be frequently torn off in the subsequent stretching step. In this case, the backing roller 2 used must be replaced with a satisfactory one or repaired. The time for this replacement or repair indicates the end of service life of the backing roll 2. According to experience, the conventional backing roller 2 has a short service life ranging from several minutes to several tens of hours depending on the kind and thickness of films to be used, the film feeding velocity and the accuracy in manufacture of the blades and roll.
When the above apparatus is operated, the backing roll 2 is cooled. As the groove (damage) in the backing roll 2 is deepened, the coolant (water) comes to stay in the groove so that blade tips of the circular blades 12 are cooled too much thus failing to form slits in the film 3.
Further, the film 3 will raise a serious problem as to excessive adhesion of the film 3 to the blade tips when it is attempted to form lateral slits in the direction perpendicular to the lengthwise direction of the film.
FIG. 20 shows lateral slits 35. In this case, a tension in the longitudinal direction (direction of arrow X) of the film 3 is exerted mainly on slit-free margins (regions respectively near the ends of side of the film) 36. Therefore, as the grooves in the surface portion of the backing roller 2 are deepened, the central portion of the film 3 is shifted from the normal position A to a position B as shown in FIG. 21. As a result, the time of contact between the blade tips and the film 3 is further prolonged so that excessive melting of the film and adhesion to the blade tips is accelerated.
On the other hand, there has heretofore been known such an apparatus for perforating synthetic resin-made films as disclosed in Japanese Pat. Appln. Laid-Open No. Sho 58-7326. This conventional apparatus has a rotary heating roller provided on the peripheral surface with projections. In this apparatus, a rotary backing roller for backing a film is arranged opposite to the rotary heating roller and is provided on the peripheral surface with grooves which correspond to said projections. There is formed an adiabatic space between the projection and the groove corresponding thereto. Further, this known apparatus has a driving unit (ascent/descent-operating unit) for driving the rotary heating roller. When a film is to be passed without being perforated, the rotary heating roller is displaced at a standby position by the use of said driving unit. When a film is to be perforated, the rotary heating roller can be moved close to the rotary backing roller so that the projections on the peripheral surface of the rotary heating roller contact with (or abut against) the film.
In the case where the grooves of this apparatus are applied to the backing roller of the previously described apparatus, the problem that satisfactory slits are not formed in the film due to the various errors or defects, and the slit-formed film is less torn off by stretching when subjected to stretching in the subsequent step. Further, the service life of the backing roller is not shortened.
However, in the case where it is assumed that the above technique is applied to an apparatus for forming longitudinal slits and grooves are formed in the peripheral surface of a backing roller and in the peripheral direction thereof, a film will partially droop into the grooves when blade tips abut against the film whereby satisfactory longitudinal slits cannot be formed. The film droops into the outermost grooves deeper than the inner ones. Even if the blade tips are heated to melt the film, there will be still raised a problem as to the drooping of the film depending on the relationship between the slit forming speed and the film melting speed. It would be thought of at this point to prevent the film from drooping into the grooves by increasing the tension applied to the film in the longitudinal direction thereof and strengthening the frictional power between the film and the backing roller, but the prevention cannot surely be achieved. In the case where it is assumed that the above technique is applied to an apparatus for forming lateral slits and the backing roller is provided on the peripheral surface with grooves in the axial direction of the backing roller, no tension can be applied to the film in the longitudinal direction. Thus, the film cannot be prevented from drooping into the grooves so that it is difficult to form slits. In addition, there is raised a problem that the central portion of the film is shifted to the rotary blade assembly, as shown in FIG. 21. If the driving unit of the above conventional apparatus is assumed to the applied to this apparatus, the blades for forming lateral slits will not enter the grooves of the roller when the rotary blade assembly is caused to approach the backing roller, thus damaging the blades.